Of course medium. I've recently published a post on medium related to an open source note taking webapp on medium - https://medium.com/@vivekweb2013...
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I believe there are more readers in Medium than Substack. In fact, monthly traffic and volume are top-notch!
Besides that, most writers and users are publishing great and read-worthy articles in it, so I'd choose Medium :)
Medium will help you in exposing your content more naturally, increasing your visibility to their daily audience but the way it is faster to publish on Substack, is just something, in my humble opinion.
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I think the medium is best and it is perfect for early-stage startups
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